Developmet of Phono-mediated Terahertz Detectors using Superconducting Tunnel Junctions

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First, we fabricated a phonon-mediated terahertz detector using superconducting tunnel junctions (STJs). The layer structure is Nb/Al/AlOx/Al/Nb fabricated on LiNbO_3 and LiTaO_3 substrates. The sizes ranged from 5 to 200μm square. The subgap leakage current at 4.2, 1.5 and 0.35K showed the similar values with the STJs fabricated on conventional Si and sapphire substrates. This means that the new substrates can be used in high-performance terahertz detector. Then, we designed and made a new optical components for our cryostat. Using this setup, we irradiated pulsed terahertz waves and read the signals from the detectors through a charge-sensitive preamplifier. We successfully detected the corresponding pulse signals for the first time. In addition, we confirmed that the rise time of the pulse signal was relatively long, showing that the detection process was consistent with the phonon-mediated detection.Next, we analyzed the detected pulse shapes and found that one pulse included two different pulses with the relatively short and long risetime, respectively. They were distinguished by applying the time derivative of the output charge. Combined this result with X-ray measurements, they were identified as the events directly absorbed with the superconducting electrode and the phonon-mediated events. We also checked the frequency response and confirmed the flat spectral response in 1-2 THz.Moreover, we introduced (1) thicker substrates for increasing the efficiency, (2) a series array detector to increase the detection area, and (3) the STJs with larger bottom electrode and smaller top electrode. In (1), we observed the increase of the efficiency. In (2), we could not get the greater signal. In (3), we fabricated STJs with the different size from 25-70μm and evaluated the mean free path of the quasiperticles inside the superconducting electrode.